The present invention relates to a new class of materials possessing a substructure of oriented aggregates of suspended magnetic particles. This substructure self-assembles under the influence of an external magnetic field, and induces a wide range of mechanical, dielectric, magnetic, and optical properties. In a particularly useful subclass of these materials, the magnetic particles are electrically conducting or are coated with a conducting layer, and the columnar concentrations are just dense enough to form a continuously conducting path through the material. These new materials enable a broad range of sensor devices and other applications.
The conduction of electricity in materials comprising a particulate conducting phase dispersed in a nonconducting medium have been of scientific and practical interest for some time. Such materials as conductive inks, some forms of conducting polymers, and static elimination materials have long used such dispersions to provide conductivity to conventionally non-conducting elements.
Prior art primarily considers applications of a composite mixture of conducting particles essentially uniformly distributed in a nonconducting medium. Roughly speaking, one expects the conductivity of a composite mixture to increase as the volume fraction of the dispersed conducting phase increases (i.e., as more conductive particles are introduced into the mixture). This is true, but the bulk conductivity of the composite is not simply proportional to the volume fraction of the dispersed phase.
If the volume fraction of conducting particles is small, then on average each particle will be surrounded by a layer of the nonconducting medium, so that the individual particles do not touch each other. In this case, the total conductivity of the composite remains very small. Alternatively, if the volume fraction of conducting particles is large, then on average each conducting particle will make effective electrical contact with a sufficient number of neighboring particles that the bulk conductivity of the composite will be large.
There is a volume fraction, whose exact value depends on the shape and size distribution of the conducting particles, near which the bulk conductivity of the composite rapidly increases by many orders of magnitude. Below this critical volume fraction, conductive paths within the composite extend only short, microscopic distances, being interrupted by particles in poor electrical contact. The result is low bulk conductivity. Above this critical volume fraction, bulk electrical conduction is dominated by conducting particles which are essentially in direct physical contact, giving the composite high bulk conductivity.
Near the critical volume fraction for bulk conduction, there are many conducting paths that are only interrupted by a few instances where current conduction must go through particles which are nearly, but not quite in contact. Small changes in the particle volume fraction can complete many of the paths, making the conductivity of these materials very sensitive to such changes.
Applications exist for such essentially uniform composite materials. An example appears in U.S. Pat. 5,574,377, in which a chemical sensor is implemented by measuring the electrical resistance of a composite material formed of a gel-like polymer containing dispersed conducting particles with volume fraction near the critical volume fraction. The sensor material has large conductivity in the absence of external chemicals. However, the sensor material (more particularly the nonconducting polymer) swells when in the presence of certain organic solvents. Such swelling increases the gaps between particles, thereby driving a large reduction in the bulk conductivity of the sensor material. Such chemical sensors can be quite sensitive if the proper volume fraction is achieved in the sensor material.
Despite the clear potential for using such near-critical composite materials for a variety of functions, practical applications are limited by prior art fabrication technology. It is very difficult to disperse conducting particles uniformly in a nonconducting medium. Exceedingly small changes in process conditions, or simply random variations in the local volume fraction of the conducting particles, can reduce or destroy the desired material response.
Thus, near-critical composite materials made using conventional technology cannot be routinely applied to most applications unless a great deal of effort is taken to control and then characterize the composite. Numerous samples must be typically made under slightly varying conditions, and the samples then individually characterized in a search for individual pieces having the proper bulk properties. When such composites can be used, the device or mechanism thereby enabled usually requires individual calibration.
There is thus a longstanding need for composite materials comprising conducting particles which can be reliably manufactured to exhibit precise and predefined conducting properties.